In the recent years, the electronics industry has gained a great advance, and concurrently various kinds of rare earth elements have became utilized in a wide variety of applications. Particularly, in the field of "Fine Ceramics" industry, various kinds of the oxides of rare earth metals have overtaken some important roles. Especially, yttrium oxide is now calling great attentions, because the yttrium oxide can be admixed with zirconia (i.e., zirconium oxide) for the purpose of controlling the phase-transition of zirconia ceramic products. In these years, it has been found that the method of producing the oxides of rare earth metals, including the yttrium oxide, can be carried out by hydrolysis of such an alkoxide of a rare earth metal represented by the following general formula EQU M(OR).sub.3 (I)
wherein M denotes a rare earth metal element such as scandium, yttrium, lanthanium, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutecium; and R denotes an alkyl group, preferably an alkyl group containing 1 to 10 carbon atoms.
In the prior art, different processes of preparing the rare earth metal alkoxide of the general formula (I) which is useful as the intermediate materials for use in the production of the rare earth metal oxides are known, including the under-mentioned processes:
(1) A first process comprising reacting anhydrous scandium chloride with dry methanol (see the "Zeitschrift Anorganische und Allgemeine Chemie" 325 (1-2) pages 67-71, (1963)). PA1 (2) A second process comprising reacting an anhydrous rare earth metal chloride with an alkali metal alkoxide in an inert, dry organic solvent (see the "Proceeding Nucleus Radiation Chemical Symposium" pages 15-19 (1964); the "Chemistry and Industry" page 120 (1963); ditto, Vol. 9, pages 382-383 (1965); ditto, Vol. 32, pages 1379 (1966); the "Chemische Berichte" Vol. 93, pages 652-657 (1960); the "Inorganic Chemistry" Vol. 5(3), pages 342-346 (1966); and U.S. Pat. No. 3278571 specification, and PA1 (3) A third process comprising reacting a rare metal element with an alkanol in the presence of mercuric chloride as catalyst (see the "Inorganic Chemistry" Vol. 5(3), pages 342-346 (1966); and U.S. Pat. No. 3278571 spec.).
With all the above-mentioned processes of the prior art, however, these processes suffer from such disadvantages that the desired rare earth metal alkoxides can be obtained only in a poor yield and also contain some quantities of detrimental impurities. Consequently, the rare earth metal oxide products as obtained from such rare earth metal alkoxides of a low purity are also containing considerable quantities of the impurities. It follows that some operations of the purification are required in order to yield such a rare earth metal alkoxide product of a high purity which can provide a high purity grade of the rare earth metal oxide products. In the consequence, the cost for the production of the rare earth metal alkoxide of a high purity is inevitably expensive, rendering the resultant rare earth metal oxide products expensive, too. The first and second processes of the prior art essentially require the starting rare earth metal chloride employed should be anhydrous, as otherwise the desired reaction substantially cannot proceed. Besides, amongst the above-mentioned processes of the prior art, the first and second processes of the prior art are particularly disadvantageous in that the anhydrous rare earth metal chlorides employed as the starting material are normally of a very much highly hygroscopic property and are very difficult to be handled in a large quantity in commercial practice. The above-mentioned second process of the prior art is also disadvantageous in that the rare earth metal elements employed as the starting material are normally very expensive and the necessary reaction cannot be finished without conducting the refluxing of the reaction mixture under heating for a long period of time, so that this process is very time-consuming and is not suitable as a commercial method which is to be carried out in a large scale. Furthermore, all the first, second and third processes of the prior art as mentioned above involve the use of the chlorides as the starting material or as the reaction reagent or as the necessary catalyst, and usually a non-neglectable quantity of the chloride anion should necessarily be migrated into the intermediate products and even into the final products as the detrimental impurity which is very hardly to be eliminated through simple procedures of purification. This seems largely due to that a slight quantity of the rare earth metal chloride is soluble in organic solvents such as aromatic hydrocarbons, e.g., benzene, toluene and xylene.
In these circumstances, we, the inventors, have made extensive researches on the process of producing the rare earth metal alkoxide in an attempt to provide a new improved process which can be free from the aforesaid disadvantages of the prior art processes. As a result, we have now found that when an anhydrous rare earth metal carboxylate is employed as the starting material in place of the anhydrous rare earth metal chloride which had been employed in the prior art processes and when this anhydrous rare earth metal carboxylate is reacted with an alkali metal alkoxide, there can be produced the rare earth metal alkoxide of a higher purity in a higher yield than by the prior art processes and the rare earth metal alkoxide as produced can easily be recovered. On the basis of these findings, we have finished this invention.